Formed fiberboard containers, such as paper plates and trays, are commonly produced either by molding fibers from a pulp slurry into the desired form of the container or by pressing a paperboard blank between forming dies into the desired shape. The molded pulp articles, after drying, are fairly strong and rigid but generally have rough surface characteristics and are not usually coated so that they are susceptible to penetration by water, oil and other liquids. Pressed paperboard containers, on the other hand, can be decorated and coated with a liquid-proof coating before being stamped by the forming dies into the desired shape. Large numbers of paper plates and similar products are produced by each of these methods every year at relatively low unit cost. These products come in many different shapes, rectangular or polygonal as well as round, and in multicompartment configurations.
Pressed paperboard containers tend to have somewhat less strength and rigidity than do comparable containers made by the pulp molding processes. Much of the strength and resistance to bending of a plate-like container made by either process lies in the side wall and rim areas which surround the center or bottom portion of the container. In plate-like structures made by the pulp molding process, the side wall and overturned rim of the plate are unitary, cohesive structures which have good resistance to bending as long as they are not damaged or split. In contrast, when a container is made by pressing a paperboard blank, the flat blank must be distorted and changed in area in order to form the blank into the desired three dimensional shape. Score lines are sometimes placed around the periphery of blanks being formed into deep pressed products to allow the paperboard to fold or yield at the score lines to accommodate the reduction in area that takes place during pressing. However, the provision of score lines, flutes, or corrugations in the blank may result in a formed product with natural fault lines about which the product will bend more readily, under less force, than if the product were unflawed. Shallow containers, such as paper plates, may also be formed from paperboard blanks which are not scored or fluted, but the pressing operation will cause wrinkles or folds to form in the paperboard material at the rim and side walls of the container at more or less random positions; these folds, again, act as natural lines of weakness within the container about which bending can occur.
In the common process for pressing paperboard containers from flat blanks, a sheet or web of paperboard is cut to form the blank--a circular shape for a plate--and the blank is then pressed firmly between upper and lower dies which have die surfaces conforming to the desired shape of the finished container. The paperboard web stock is usually coated with a liquid-proof material on one surface and may also have decorative designs printed under the coating. The surfaces of the upper and lower dies have typically been machined such that, when they begin to compress the shaped paperboard blank between them, the die surfaces will be generally spaced uniformly apart over the entire surface area of the formed paperboard. The lower die is spring mounted to limit the maximum force applied to the paperboard between the dies; and this force is distributed over the entire area of the paperboard if the spacing between the dies is uniform. In practice, the machining of the dies is such that random high and low spots are commonly formed on the die surfaces, resulting in random, localized areas of the paperboard which are highly pressed while other areas are unpressed. The dies are also generally heated to aid in the forming and pressing operation. Paperboard plates produced in this manner have good decoration quality and liquid resistance because of the surface coating, and are suited to high production volume with resulting relatively low unit cost. However, as noted above, the plates suffer from a lower than desired level of rigidity and are subject to greater bending during normal household use than is perhaps most desirable.
While problems with the rigidity of pressed paperboard containers have long been known, there has heretofore been limited success in improving the rigidity qualities of these products in a commercially practical manner. One example of a process intended to increase the rigidity of pressed paper plates is shown in the patent to Bernier, et al., U.S. Pat. No. 3,305,434. A process is disclosed therein in which paperboard having very high moisture content, in the range of 15% to 35% by weight, is pressed between heated forming dies which are specially designed to allow escape of the water vapors driven off during the pressing operation. The paperboard blank stock is thus relatively soft and easily formed into shape. Distortion of the shape of the soft and flowable fiberboard is prevented by driving the forming dies to a stop at which the surfaces of the dies are uniformly spaced apart a distance approximately equal to or slightly less than the desired thickness of the formed container. The shaped fiberboard material dries under the heat and pressure applied by the dies and the fibers within the material build up internal bonds upon drying which help to maintain the strength and rigidity of the deformed portions of the paperboard material. The apparent limitations of such a process are the complex dies required to allow release of the water vapors from the pressed fiberboard, handling problems with high moisture fiberboard, and slower production times required because of the time necessary to allow removal of the water vapor from the paperboard during the pressing operation, thereby all contributing to increased production costs.